The basic operation and structure of trunking communication systems and cellular telephone communication systems (cellular communication systems) are known. Trunking communication systems typically comprise a communication channel controller, a limited number of repeaters that transceive information via communication channels, and a plurality of communication units which may be mobile vehicle radios and/or portable radios. Of the communication channels, one is typically chosen to be a control channel. The control channel typically transceives operational information between the communication channel controller and the plurality of communication units such that, for example, the plurality of communication units can access the communication channels. Typically, the trunking communication system has a relatively large geographic coverage area, depending on the environment that the trunking communication system is located, the coverage area may be approximately 35 miles in diameter.
A cellular communication system typically comprises a mobile telephone switching office (MTSO), a plurality of cells, a limited number of communication channels, and a plurality of communication units which may be cellular telephones. Each of the plurality of cells comprises some of the of a limited number of communication channels, wherein one of the communication channels is designated as the control channel for that cell. The control channel transceives operational information between the plurality of communication units within the cell and the MTSO such that the communication units can place telephone calls via a phone system. Typically, the coverage area of each cell is relatively small in comparison with that of a typical trunking system. For example, a typical cell coverage area is approximately two miles in diameter. Because an individual cell coverage area is relatively small, the communication channels may be reused, at least once, within a geographic region of approximately the same size as a trunking communication system.
Due to the different configurations of a trunking communication system and a cellular communication system, a typical communication unit, without two receivers and two transmitters, cannot operate in both communication systems. (Having two receivers and transmitters adds considerable expense to manufacturing and purchasing of a communication unit, thus, for practical reasons, most communication units have only one receiver and one transmitter.) Recent advances in technology have expanded the bandpass of the receivers and transmitters in communication units such that a communication unit is able to transceive in either system. (Note that presently the FCC has designated for a trunking communication system, the frequency range of 806 to 821 Mhz as the transmit region and the frequency range of 850 to 865 Mhz as the receive region, and, for a cellular communication system, the frequency range of 824 through 849 Mhz as the transmit region and 869 to 894 Mhz for the receive region.) However, a communication unit with one receiver and one transmitter, is limited to operating in one system or the other due to technological limitations of the receiver and transmitter. Within the receiver, a mixer mixes a received signal with a local oscillator to produce an intermediate signal. The intermediate signal is passed through a bandpass filter which substantially filters out all other signals including those transmitted by the other communication system. The transmitter is limited for similar reasons.
Another limitation that impedes communication units from operating in both systems is that, in many cases, a trunking communication system and a cellular communication system within the same geographic area may be manufactured by different manufacturers. This presents a limitation in that typically one manufacturer does not know, or cannot use, the signalling protocol of the other, which makes accessing that system very difficult.
Therefore, a need exists for a method that will allow a communication unit having one receiver and one transmitter to operate in either a trunking communication system or a cellular communication system, wherein switching from one system to the other will be done transparent to a user of the communication unit and regardless of the manufacturers of the systems.